With these contracts, the best way to start them is by ordering over the internet or over the phone so that you are covered by distance selling regulations and can, if reception isn't good, cancel with no penalty within 7 days.
Nine months on, you should write a couple of letters explaining the problem to Three and then, after you've given them a reasonable chance to put things right, cancel the payments by contacting your bank and write another letter stating what you've done and why. If they later come chasing you for more money, you simply show a copy of the letters, explain that the problem was never sorted out and, if necessary, say that you would be happy for the matter to be put before a magistrate for him/her to decide if you should pay.
You are a long long way from being the first to have problems with Three's coverage.